The present invention relates to the continuous casting of aluminum (including aluminum alloys), and in particular it relates to continuous casting in so-called "hot top" molds.
In continuous casting practice, molten metal is poured into an open-ended mold, the lower end of which is initially closed by a stool or platform, which is progressively lowered in step with the pouring of the metal. In normal continuous casting practice, the wall of the casting mold is cooled, so that a solid skin of metal forms in contact with the mold wall at the level of the surface of the pool of molten metal in the mold. For practical reasons, it is preferred in the commercial production of ingots to maintain the surface of the molten metal at least 11/2 inches (3.8 cms) above the bottom margin of the mold, below which the surface of the emerging ingot is directly cooled by contact with coolant water. Unless that step is adopted there is always a risk of a run-out of molten metal from the bottom of the mold arising from an unexpected fluctuation of the molten metal level. Although it is possible under laboratory conditions to cast with the level of metal lower in the mold with appropriate casting speeds, such operation poses intense difficulties of control with a casting table having 40-60 molds.
It has long been understood that the production of metal ingots in conventional molds, in which the whole surface of the mold wall is chilled, may result in ingots having surface defects. The two commonest forms of surface defect are referred to as "cold shuts," which appear on the ingot in the form of transverse ridges, and "bleeds" or liquation where low melting components of the molten alloy can flow from the pool in the center of the ingot through the interdendritic channels in the shell to the ingot surface in the interval between the formation of the skin at the level of the top surface of the pool and the level at which the peripheral surface of the ingot comes under the chilling influence of the coolant applied to the surface of the ingor below the mold. In this interval, the surface of the skin is subjected to very little cooling since the solid skin contracts away from the mold wall and the transfer of heat from the skin to mold wall is minimal except at or slightly below the meniscus of molten metal at the surface of the pool. It has long been apparent that the "cold shuts" on the surface of the ingot are due to instability of the metal meniscus due to contraction of the molten metal on solidification, related to overcooling of the ingot.
It has already been observed that the coolant applied to the solidified surface of the ingot as it emerges from the mold exerts effective chilling action on the skin for a distance of about 1 inch (2.5 cms) above the level at which it is applied, this distance varying somewhat with change of casting speed. It has been postulated that by employing thermal insulation in the mold so as to hold the metal out of contact with the chilled mold wall until it reached a level at which the skin could be solidified almost solely by the heat withdrawn by means of the sub-mold coolant (coolant applied directly to the surface of the ingot below the mold) smooth-surfaced ingot could be produced. Although smooth-surfaced ingot can be produced in that way, the results are not consistent.
It has been suggested more recently that smooth-surfaced ingots can be produced in hot-top molds by correlating the depth of chilled mold with the dropping rate of the casting. However, this arrangement has also been found to provide inconsistent results, and in practice, when following that teaching, ingots having various forms of surface defect, as well as some smooth-surfaced ingots, have been produced.
In "hot top" mold casting, as presently practiced for the continuous casting of aluminum (including aluminum alloys), a head of molten metal is held in a thermally insulated "hot top" placed over the top of the chilled casting mold and acting as an upward extension of such mold. The "hot top" has a flat bottom end surface and is usually of somewhat less diameter than the chilled mold.